<!DOCTYPE html>
<html lang="en" style="font-size: 20px">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style> 
        #app{
            width: 500px;
            height: 400px;
            border:solid 1px rgb(141, 41, 41);
            font-size: 14px;
        }

        .one{
            width: 200px;
            height: 100px;
            border:solid 1px #345;
            /* em 相对于父级元素 font-size */
            font-size: 1em;
        }

        .two{
            font-size: 0.8rem;
            width: 20rem;
            height: 3rem;
            border:solid 0.2rem #000;
        }
    </style>
</head>
<body>
    <div id="app">
        <div class="one">
            我们升级了, 成了农民工选手, 以后他们不敢拖欠我们的工资了
        </div>
        <hr>
        <div class="two">
            阿富汗飞机场出现了严重的拥堵, 场面一度混乱
        </div>
    </div>
</body>
</html>